This invention relates to the art of video display of images representing data characters or symbols or other graphical images.
The invention is particularly applicable for use in conjunction with a video display system employed in text editing and the like and will be described in conjunction therewith; although, it is to be appreciated that the invention has broader applications as it may be used in various video displays wherein it is desired to modify one video image with another.
Video display systems having the capability of displaying data characters and for modifying video characteristics of such data characters are known in the art. Examples of prior art patents on the subject include the U.S. Pat. to R. C. Williams Nos. 3,895,374, 3,895,375, and 3,896,428. These patents each teach video modification of the display of a data character such as inversion in appearance, intensification, and underlining. One or more of these modifications may be made to a data character. The apparatus disclosed in those patents by employs a data stream wherein the data characters are encoded in binary form and are preceded by a binary encoded attribute character which commands the attribute or modification to be made to the data characters following the attribute character. The attribute character is a multi-bit character with each bit being representative of one of several attribute modifications which may be made.
A notable problem with such display systems as described above is that logic circuitry must be employed to decode each multi-bit attribute character to decide which modification or modifications are to be made to the visual images representing the data characters following the attribute character. The video image of each data character is formed by obtaining video instructions for that character from a look-up table, such as a read only memory. These instructions are supplied to a video generator which may include a T.V. raster scan for forming segmental dot patterns or dot slices of each of a plurality of characters forming a character line. Several scans are made until each character of the character line has been formed. Logic circuitry must be employed to respond to the attribute data character to make the appropriate video modifications to the images of the data characters being formed. The complexity of such logic circuitry is at one level with such enhancements as intensification or inversion in appearance since each dot position is enhanced in the same fashion. Consequently, only one piece of information is required by the logic circuitry. The logic circuitry required becomes far more complex for such attributes or enhancements as strike-through, underlining, or cross hatch because each of these require that the dot pattern of the enhancement be combined with the dot pattern representing the visual image of the data character. Moreover, such complex logic circuitry to provide these attributes or enhancement would not be programmable in the field. Consequently, the user of such equipment would be limited to those attributes or enhancements provided by the terminal manufacturer.
Another disadvantage of such video display terminals as that discussed above is the inability to easily create graphical images which combine the visual characteristics of a data character from one library or set of data characters such as the English alphabet, with the visual characteristics of other data characters or graphical images from an independent, nonrelated library or set of graphical images. For example, an editor may want to change the meaning of the letter "O" taken from the English alphabet. If a horizontal bar could be placed through the letter "O", then the meaning of the letter "O" has been changed to theta, a character taken from the Greek alphabet.
Thus it would be desirable to provide a plurality of storage libraries each storing video instructions for forming a set of symbols or data characters or other graphics and the like such that when a particular symbol or character or graphics is called for, its video image may be combined with the video image taken from a different library to form a graphical image having the combined video characteristics of the various symbols, characters, or graphics taken from the various libraries.